Conventional spectroscopic imaging systems are generally based on the application of high resolution, low aberration, lenses and systems that produce images suitable for visual resolution by a human eye. These imaging systems include both microscopic spectral imaging systems as well as macroscopic imaging systems and use complex multi-element lenses designed for visual microscopy with high resolution aberrations optimized for each desired magnification. However, transmitting illumination through such complex lenses attenuates the incident beam and creates spurious scattered light.
Further, each lens magnification results in a particular collection angle for the scattered light. Generally, at lower magnification the collection efficiency is strongly reduced as the focal distance increases. Consequently, the lens must be placed further away from the sample. For macro-systems (i.e., systems needing a broader view of the larger sample rather than a high magnification of a smaller portion of the sample), the reduced collection aperture severely limits the collected signal. The need for high collection efficiency may be critical for spectroscopic imaging at all distances.
Much of the optical signal detected in the conventional systems is dramatically reduced because of the system configuration and the need to maintain high resolution by removing optical aberrations. Conventional systems have been largely conceived based on the premises and the requirements of optical microscopy. Namely, the need to present a high resolution, zero-aberration, image to the operator who uses visual inspection to perceive the image. In addition, conventional micro-Raman systems achieve their high spatial resolution through the focus of the laser beam to a diffraction-limited spot by the microscope's objective lens. These design premises and system configurations limit the light delivery in conjunction with the collection efficiency of the spectroscopic imaging system.
Finally, design premises based on resolution and throughput requirements for spectral imaging have not been changed as components have been adopted or selected from commercial optical systems. Illumination through such optical systems produces attenuation (reduced signal) and internal scattering (higher background noise) which are detrimental to the system's performance. Thus, there is a need for a low cost, high throughput and efficient chemical imaging system.